Low temperature light off ammonia oxidation

ABSTRACT

Platinum-rhodium and platinum-palladium-rhodium gauzes bearing a high surface area coating (in excess of 50 cm 2  /g) of platinum ease initiation of ammonia oxidation.

Start-up problems have long plagued manufacturers of nitric acid. Intypical operations, catalytic oxidation of ammonia in heated air isinitiated over platinum containing gauze using a hydrogen torch whileoperations are controlled manually based upon visual observation of thegauze. Not only is it possible to damage the catalyst gauze with thehydrogen torch but also unconverted ammonia is often nonselectivelydecomposed on the recovery gauze during start-up due to by passing ofammonia through the catalyst gauze resulting from wide variation intemperature between the hot portion of the gauze heated by the torch andremote cooler regions. Even worse, mixing of NO_(x) and unconvertedammonia in the getter gauze can lead to unfavorable reactions which maydamage some types of very high efficiency recovery gauzes particularlyin plants operating at very high throughputs and pressures. Thus, atbest, traditional methods of starting up plants using conventionalcatalyst gauzes have been tricky and time consuming while wastingsignificant amounts of ammonia, but, in some cases, also resulting inexpensive damage to catalyst or recovery gauzes.

Methods of alleviating these problems have been sought widely. Onepurported solution (U.S. Pat. No. 4,435,373) involves pads made byagglomeration of randomly oriented platinum group metal fibers producedby melt extraction or melt spinning. It is said that these pads exhibit. . . "more rapid light-off, better conversion efficiency and increasedlife . . . ". The alloys used in these pads are 90% Pt:10% Rh and 90%Pt:5% Rh:5% Pd alloys. In example 3 of this Patent, the surface area ofmelt spun 90% Pt:10% Rh fibers is said to be 8.3 m² /g, the surface areaof melt spun 90% Pt:5% Rh:5% Pd fibers is said to be 13.3 m² /g ascompared to conventional 90%Pt:10% Rh wire having a surface area of 6.9m² /g. These figures are almost certainly intended to be in cm² /g as6.9 m² /g is at least four orders of magnitude larger than the surfacearea of conventional platinum rhodium wire.

In British Patent No. 1,426,875, ammonia combustion is initiated overcobalt oxide containing catalysts by homogeneously combusting hydrogenupstream of the catalyst and passing the combustion products over thecatalyst bed thereby mitigating the difficulties of localizeddeactivation and unsatisfactory propagation of reaction through thecatalyst which are said to result from heating the catalyst locally witha hydrogen flame to initiate oxidation. A similar method is disclosedfor plants using catalytic gauzes in U.S. Pat. No. 2,185,607.

In USSR Inventors Certificate No. 858905, exothermic heterogeneouscatalytic oxidation of from 1 to 3.5% hydrogen in air on the surface ofthe catalyst gauzes is used to provide uniform heating of the gauzesprior to passage of ammonia and air mixture over the gauzes. The mixtureof hydrogen and oxygen contacted with the gauzes is said to be at atemperature of 140°-250° C.

Handforth and Tilley, Catalysts for Oxidation of Ammonia to Oxides ofNitrogen, Industrial and Engineering Chemistry Volume 26 no. 12 pp.1287-1292 reference reportedly unsuccessful efforts to use refractorybase metal substrates clad with platinum group metal for ammoniaoxidation which were said to fail as volatilization removed the activematerial and the underlying metal quickly poisoned the coating.

U.S. Pat. No. 1,321,376, Jones and Parsons suggests the use of "platinumgauze which has been "platinized" or coated with platinum black [i.e.platinum oxide] or platinum sponge by depositing upon it more platinumfrom solution".

It is an object of the present invention to provide a catalyst gauzewhich will light off quickly and easily at lower temperatures than theknown gauzes or pads and, also, a method of initiating uniform ammoniaoxidation over these gauzes starting from more accessible temperaturesobtainable by compression of air so that preheaters and burners are notrequired.

SUMMARY OF THE INVENTION

Ammonia is oxidized easily at lower temperatures over platinumcontaining gauzes bearing a platinum coating having a surface area inexcess of about 50 cm² /g. These gauzes may be heated to a temperaturewhich is sufficient to initiate ammonia oxidation by passing hydrogen inair over these gauzes at temperatures below 75° C., most preferably attemperatures of less than 50° C.

CROSS REFERENCES

This application claims a catalyst gauze bearing a high surface areacoating of platinum thereupon and methods of using that gauze inoxidation of ammonia.

Copending application Ser. No. 06/887,578, filed Aug. 6, 1989, claims amethod of low temperature start-up of ammonia oxidation plants bypassing hydrogen and oxygen over these high surface gauzes therebyheating them to temperatures which are sufficient for catalyticoxidation of ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the decline in light off temperature observed withgauzes having increased surface area at constant flow rate.

FIG. 2 illustrates predicted and observed variations in ignitiontemperature with flow rate for coated gauzes of the present invention.

FIG. 3 illustrates the ignition temperature of hydrogen observed overgauzes of the present invention.

FIG. 4 illustrates the improved selectivity and conversion efficiencyobserved with gauzes of the present invention during the first daysfollowing start-up.

FIG. 5 illustrates the increased production rate of nitric acidexperienced with gauzes of the present invention during the first fewdays following start-up.

FIG. 6 illustrates the variation in surface area of coated gauzes withthe temperature at which the coating precursor is decomposed and alsoillustrates the effect of decomposition temperature on ignitiontemperature.

FIG. 7 is a photomicrograph of a high surface area platinum coating of agauze of the present invention.

FIG. 8 is a photomicrograph of a non-uniform coating prepared with anaqueous chloroplatinic acid solution.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The ammonia-air mixture in commercial nitric acid plants is commonlyignited using a hydrogen torch directed to a spot on the gauze which hasbeen preheated with compressed hot air. After the spot begins to glow,ammonia is gradually added at an increasing flow rate until ignition isobserved. Thereafter, heat conduction from the hot spot brings theremainder of the gauze up to the ignition temperature. Difficulties inproperly manipulating the hydrogen torch are common especially for largegauzes. Too frequently, the torch burns a hole in either catalyst orrecovery gauze while in other cases, initial ignition is slow or thespread of ignition is slow and uneven. We reduce these problems greatlyby using catalyst gauzes with improved ignition characteristics whichalso protect the getter gauzes by reducing ammonia by pass duringstart-up. In addition, ignition stability is also significantly improvedso that once ignition is obtained on a gauze, it can be sustained ateven lower temperatures than the initial ignition temperature, in somecases even when the temperature of the inlet stream approaches ambient.This wide temperature range between ignition and extinction areimportant advantages for stable reactor operation during and followingstart-up. Further, these gauzes may be preheated by catalytic combustionof hydrogen occurring at relatively low temperatures usually obtainablein the plant flow stream.

Catalyst gauzes of the present invention are produced by forming anextremely catalytically active coating on conventional ammonia oxidationcatalyst gauzes. Such gauzes typically consist of platinum group metalalloys primarily 90% Pt:10% Rh and 90% Pt:5% Rh:5% Pd and are typicallywoven meshes of about 0.003 of an inch diameter wires at about 80 wiresper lineal inch. Other combinations of mesh and wire diameter may beused to advantage. For a detailed report, see Roberts and Gillespie,"Estimation of Platinum Catalyst Requirement for Ammonia Oxidation" 45Advances in Chemistry Series No. 133, Chemical Reaction Engineering IIpage 600-611. See also U.S. Pat. No. 3,660,024.

The catalytic coating may be applied by a variety of methods producingextremely high surface area coatings of substantially pure platinum ontothe above mentioned alloys providing decreased light off temperatureupon a strong thermally stable base.

Preferred high surface area coatings may typically be formed bydecomposing platinum metal precursors on the catalyst gauze after it hasbeen woven and cut to shape. For ease of manufacturing, the preferredplatinum metal precursors are aqueous admixtures bearing thermallydecomposable platinum metal compounds although other solvents includinghydrocarbons, particularly butanol, may also be used to easily providehigh uniformity coatings. The preferred method of applying thesecoatings to the gauze is by spraying, with electrostatic spraying beinggreatly preferred. The precursor admixture may also be applied bydipping the gauze in the admixture, precipitation methods such aselectroless deposition, electroplating, electrodeposition, or brushingor painting the admixture on, much as paint is applied with a brush.More esoteric methods such as chemical vapor deposition, sputtering andother forms of physical vapor deposition may also be used.

As shown in FIG. 1, ignition temperature correlates well with surfacearea of platinum coated gauzes prepared from solutions of chloroplatinicacid in either normal-butanol or water with the ignition temperatures ofthe water based coatings being higher however than those of the butanolbased coatings, probably due to difficulty of obtaining a uniformcoating with aqueous based systems, this difficulty being manageable byproper use of surfactants, such as Igelpal CO-630, Colloids 211 and 209which do not poison platinum. The preferred platinum coated gauzes ofthe present invention are characterized by a surface area preferably inexcess of about 50 cm² /g, and more preferably in excess of about 250cm² /g and most preferably in excess of about 350 cm² /g. The platinumloading is preferably at least about 0.5 grams of platinum metal persquare meter of bulk gauze area coated essentially uniformly onto thegauze. By bulk gauze area, we mean not the actual or total surface areaof the aggregate of the wires in the gauze considered for example ascylinders but rather the area of the gauze considered as a sheet ofmaterial. For example, a one meter by one meter sheet of any gauze wouldbe said to have a bulk gauze area of one square meter. More preferablythe platinum loading is at least about 3.0 g/m² and more preferably atleast about 4.0 g/m² or about 1% of the weight of the gauze. Preferredgauzes of the present invention exhibit ignition temperatures of lessthan about 275° C., preferably less than about 260° and most preferablyless than about 250° C. in a plant operating at a mass throughput ofabout 8500 lbs/ft² -hr. As the ignition temperature over catalyticgauzes depends strongly on the flow rate over the gauze, to facilitatecomparisons, throughout this application, all ignition temperatures areas measured at this standard flow rate of 8500 lbs/ft² -hr unlessotherwise specified. FIG. 2 illustrates the expected variation inignition temperature expected with variations in the flow rate as wellas the variation actually observed in a laboratory unit.

To protect the getter gauzes, it is extremely advantageous that thelight off temperature of the catalyst gauze be no more than the ignitiontemperature of ammonia on the recovery gauze. Typically ammonia willignite on high palladium content gauzes at temperatures of around 260°C. at the standard flow rate of 8500 lbs/ft² -hr,

Gauzes of the present invention are not only easily ignited with ahydrogen torch but may also be ignited by low temperature catalyticcombustion of hydrogen on the surface of the gauzes thus obviating theuniformity problems inherent in hydrogen torch ignition. While oxidationcould be initiated over previous known gauzes using a flow of hydrogenthrough the reactor, preheaters and burners were required to bring thegauzes up to the hydrogen ignition temperature of about 80° to 125° C.Since the ignition temperature of hydrogen over gauzes of the presentinvention having surface areas of about 500 cm² /g is at about roomtemperature, hydrogen can be ignited over these gauzes using the preheatsupplied by the compressor, then the amount of hydrogen increased to inthe range of from about 3 to about 5% thereby heating the gauze to theammonia ignition temperature of about 250° C. to 300° C. FIG. 3illustrates the ignition temperature of hydrogen on platinum coatedgauze as a function of the surface area of the gauze. Thus it can beseen that even gauzes with lower surface areas can ignite hydrogen atthe temperatures typically encountered in pressurized air from thecompressor.

Gauzes of the present invention not only light off easily but they alsoprovide another unexpected benefit as illustrated in FIGS. 4 and 5 inthat the NO_(x) yield during the first few days of operation seems to bemarkedly higher than with prior art gauzes. While this advantagesubsides after a few days, over the entire cycle, it can amount to aboutone quarter of a percent or more increase in yield. Unfortunately thesecoatings do not retain their high surface area during prolonged use sothat the increased yield does not persist throughout the cycle.

The most preferred method of forming the coated gauzes of the presentinvention is by electrostatic spraying in which droplets of the coatingprecursor admixture are provided with an electrostatic charge as theypass through a high resistance (around 10 mega ohm) electrode at highvoltage comprising an array of fine wires near the fluid nozzle of hespray apparatus and thus can be gathered on grounded gauze targets withhigh efficiency, even depositing on the side of the wires away from thespray gun.

The following examples are provided to illustrate various embodiments ofthe invention in a concrete fashion.

EXAMPLE 1

This example illustrates formation of high surface area coatings usingplatinum precursors carried in n-butanol. Catalyst gauzes of 90% Pt:10%Rh with 80 wires per inch and 0.0031 inch diameter were cleaned in anultrasonic bath and washed with deionized water, then dried in air.

Two coats of high surface area platinum precursor were applied to bothsides of the gauze by spraying chloroplatinic acid in n-butanol with aplatinum content of 5% by weight onto the cleaned gauze then removingthe n-butanol by passing hot air at around 120° C. over the gauze. Afterboth coats of precursor were applied, the gauze was slowly heated to,then held for 30 minutes at, 450° C. thereby decomposing the precursorto yield a high surface area platinum coating.

Similarly other gauzes were coated by applying other platinum precursorsin n-butanol as set forth in Table I which also reports the surfaceareas obtained which are from about 5 to about 120 times greater thanthat of conventional gauzes.

The measured surface areas, as obtained by cyclic voltammetry are setforth in Table I.

FIG. 7 is a photomicrogaph illustrating the high uniformity of the Ptcoating demonstrating how use of N-butanol as a solvent allows uniformcoatings presumably by virtue of its excellent atomizing and wettingcharacteristics.

                  TABLE I                                                         ______________________________________                                        Pt-coated Gauzes Prepared                                                     from Pt Salts in N--butanol                                                   Sam-                    Pt-    Surface                                        ple           Coating   Loading                                                                              Area                                           No.  Pt Salt  Method    (g/m.sup.2)                                                                          (cm.sup.2 /g)                                                                        Remarks                                 ______________________________________                                        1    Pt       Dipping   0.71   63                                                  Resinate                                                                 2    Pt       Dipping   2.27   151    HCl Washed                                   Resinate*                                                                3    CPA      Dipping   1.87   525                                            4    CPA      Dipping   3.26   225    Double                                                                        Coating                                 5    CPA      Air Brush 3.84   573                                            6    CPA      Air Brush 3.44   385    With 0.2%                                                                     Ethylene                                                                      Glycol                                  7**  CPA      Air Brush 0.29   98     With 0.2%                                                                     Ethylene                                                                      Glycol                                  8    CPA      Air Brush 2.53   65     With 0.2%                                                                     Ethylene                                                                      Glycol                                  9    CPA      Air Brush 6.65   1064   With 0.2%                                                                     Ethylene                                                                      Glycol                                  10   Pt       Air Spray 2.17   58                                                  Resinate*                                                                11   CPA      Air Spray 3.77   256                                            12   CPA      Air Spray 5.31   170                                            13   CPA      Air Spray 4.22   1065                                           14   CPA      Air Spray 5.73   1549                                           ______________________________________                                         CPA = chloroplatinic acid                                                     Pt resinate = commercially available platinum salts of a mixture of resin     acids                                                                         **precursor applied to only one side of gauze                            

EXAMPLE II

This example illustrates formation of high surface area coatings usingplatinum precursors carried in aqueous admixtures.

Catalyst gauzes of 90% Pt:5% Rh:5% Pd with 80 wires per inch and 0.0031inch diameter were cleaned in an ultrasonic bath and washed withdeionized water. After drying in air, the aqueous chloroplatinic acidadmixture with 5% Pt by weight was sprayed onto the cleaned Pt/Rh/Pdgauze. After the carrier was removed, another coating of precursor wasapplied, and the salt converted to platinum as described in Example I.Similarly, admixtures of chloroplatinic acid in water were applied toother gauzes as set forth in Table II which also reports the surfaceareas of the resulting gauzes.

                  TABLE II                                                        ______________________________________                                        Pt-coated Gauzes prepared                                                     from Aqueous CPA Solution                                                     Sam-                   Pt-                                                    ple  Pt     Coating    Loading                                                                              Surface Area                                    No.  Salt   Method     (g/m.sup.2)                                                                          (cm.sup.2 /g)                                                                          Remarks                                ______________________________________                                        1    CPA    Dipping    0.47   63       soaked                                                                        overnight                              2    CPA    Air Spray  2.15   365                                             3    CPA    Air Spray  4.64   400                                             4    CPA    Air Spray  1.04   763                                             5    CPA    Electrostatic                                                                            1.58   1228     2.5 A/ft.sup.2                                     Deposition                 for 30 min.                            6    CPA    Electrostatic                                                                            1.21   495      one coat                                           Spray                      on one side                            7    CPA    Electrostatic                                                                            5.34   514      two coats                                          Spray                      on one side                            ______________________________________                                    

FIG. 8 is a photomicrograph illustrating a typical coating obtained bythis method. The tendency of these coatings to agglomerate or thickennear wire intersections should be noted.

EXAMPLE III

As set forth in Table III catalyst gauzes of 90% Pt:5% Rh:5% Pd werecoated with high surface area platinum using the procedures of ExampleII surfactants such as Igelpal CO-630, Colloid 211, etc., were added tothe aqueous admixture to improve wetting characteristics of the Pt saltsolution and thereby to increase the uniformity of the applied coatingswhile minimizing bridging problems in wire meshes.

Table III also sets forth the surface areas obtained along with aqualitative judgement of coating uniformity for each surfactant.

                                      TABLE III                                   __________________________________________________________________________    Effect of Surfactant                                                                                       Surface                                          Sample           Coating                                                                             Pt Loading                                                                          Area                                             No. Pt Salt                                                                             Surfactant                                                                           Method                                                                              (g/m.sup.2)                                                                         (cm.sup.2 /g)                                                                      Remarks                                     __________________________________________________________________________    1   CPA/H.sub.2 O                                                                       Igelpal                                                                              Air Spray                                                                           5.89  706                                                        CO-630*                                                                       (0.5% by wt.)                                                       2   CPA/H.sub.2 O                                                                       Colloid 211**                                                                        Air Spray                                                                           4.36  353                                                        (0.5% by wt.)                                                       3   CPA/H.sub.2 O                                                                       Colloid 211**                                                                        Air Spray                                                                           4.54  672  Uniform                                               (0.5% by wt.)           Coating                                     __________________________________________________________________________     *Tradename of GAF                                                             **Tradename of North Chemical Co., Inc.                                  

Catalytic performance of catalyst gauze pack samples was evaluated asfollows using the laboratory ammonia oxidation reactor described in U.S.Pat. No. 4,497,657 which had been modified to allow for placement of athermocouple in the catalyst gauze pack.

Example IV

Ignition tests were conducted with five-ply packs of gauze samples at100 psig reactor pressure and 9000 lbs/ft² -hr throughput. Athermocouple was embedded between the second and third layers of thegauze pack to determine light-off temperature.

To measure ignition temperature of gauzes from Examples I through III,air at a fixed temperature was passed through a catalyst gauze pack andintermittently ammonia was injected to give a concentration of 2.5%ammonia in air through the pack while ammonia was being fed. If a sharptemperature rise was detected by the thermocouple, it was concluded thatthe fixed temperature of the air was at or above the ignitiontemperature of ammonia in air over the particular gauze pack in use. Ifno exotherm is detected the ammonia concentration was increased to 5% todetermine whether ignition could be obtained at this higher temperature.If not the temperature of the air stream was increased and thisprocedure repeated until ignition was detected. Ignition is usuallyaccomplished by speeding up the oxidation reaction by addition of anammonia feed and the reaction itself releases heat at the gauze surfacefaster than heat is transferred to the bulk gas stream.

Thus the ignition temperature was measured as the lowest temperature atwhich the ammonia/air mixture spontaneously started to be combusted.

Table IV reports the light-off temperature obtained with each samplewhile FIG. 1 presents these results in graphic form.

                  TABLE IV                                                        ______________________________________                                        Ignition Temperature of Pt-Coated Gauzes                                      at 8500 lb/ft.sup.2 -hr and 100 psig                                          Sample  Surface       Ignition Temp. (°C.)                             No.     Area(cm.sup.2 /g)                                                                           2.5% NH.sub.3                                                                           5% NH.sub.3                                   ______________________________________                                        Example I                                                                     I-1     63            261       258                                           2       151           254       --                                            3       525           255       250                                           4       225           247       245                                           5       573           224       218                                           6       385           248       243                                           7       98            288       285                                           8       65            266       258                                           9       1064          233       225                                           10      58            249       --                                            11      256           249       241                                           12      170           245       --                                            13      1065          232       --                                            Example II                                                                    II-1    63            276       --                                            2       365           250       --                                            3       400           263       --                                            4       763           257       --                                            5       1228          253       --                                            6       495           250       --                                            7       514           260       --                                            Example III                                                                   III-1   706           242       --                                            2       353           236       --                                            3       672           232       --                                            ______________________________________                                    

EXAMPLE V

A gauze pack consisting of five layers of Pt-coated gauze with anaverage surface area of 385 cm² /g, which were prepared as in Example I,followed by a five-ply pad of conventional 90% Pt:5% Rh:5% Pd gauze wastested to measure the NO_(x) yield profile during start-up.

Prior to beginning a run, the light-off temperature was measured to be248° C. with 2.5% ammonia feed as illustrated in Example IV. Afterignition, a gaseous stream of air containing about 11% ammonia by volumeunder a pressure of 100 psig was introduced to the laboratory reactor,described in U.S. Pat. No. 4,497,657, at a nitrogen loading of about 65tons nitrogen (calculated as ammonia) per square meter of the bulkcross-sectional area of the experimental gauze per day (i.e., 65 T(N)/m²/day). The gauze exit temperature was maintained at a relativelyconstant 860° C. while the air preheat temperature was around 200° C.

As summarized in Table V, NO_(x) yield as a function of time wasdetermined with both the experimental gauze packs as well as a typicalpack consisting of 10 layers of conventional 90% Pt:5% Rh:5% Pd gauze.

A NO_(x) yield of 92% was obtained with the Pt-coated gauze pack 20minutes after start-up and a peak NO_(x) yield of 97% was achieved afterabout 2.5 hours on-stream. In addition, it was observed that a similarPt-coated gauze pack with a surface area of 1894 cm² /g reached a peakefficiency of 98% 45 minutes after start-up, while the conventionalgauze pack required about 4 hours to reach this level, as illustrated inFIG. 4.

                  TABLE V                                                         ______________________________________                                        NO.sub.X Yield Profiles                                                       Time          NH.sub.3,                                                                            NO.sub.x Yield                                           (hr)          %      %                                                        ______________________________________                                        Run 1 Pt:5% Rh:5% Pd Catalyst (10 Sheets)                                     0.3           10.89  85.6                                                     1.4           11.35  93.0                                                     1.9           10.89  92.6                                                     3.0           10.89  98.1                                                     Pt Coated Gauze                                                               Run 2                                                                         Top 5 layers: Pt-coated gauze (Surface Area = 385 cm.sup.2 /g)                Bottom 5 layers: Conventional Gauze (90% Pt/5% Rh/5% Pd)                      0.4           10.89  92.0                                                     2.6           10.89  97.1                                                     3.6           10.89  97.7                                                     Run 3                                                                         Top 5 layers: Pt-Coated Gauze (Surface Area = 1894 cm.sup.2 /gm)              Bottom 5 layers: Conventional Gauze (90% Pt/5% Rh/5% Pd)                      0.3           11.80  87.9                                                     0.6           11.80  97.8                                                     1.5           11.80  98.9                                                     2.8           11.80  98.5                                                     ______________________________________                                    

EXAMPLE VI

A commercial scale catalyst gauze pack was prepared by air-assistedspraying of 5% chloroplatinic acid/n-butanol solution as set forth inExample I.

This gauze pack consisting of the top five layers of Pt-coated gauzefollowed by seventeen layers of conventional 90% Pt:5% Rh:5% Pd wasevaluated in a commercial plant operating at:

    ______________________________________                                        Operating Pressure  125 psig                                                  Air Preheat Temp.   270° C.                                            Ammonia Feed        10.1%                                                     Gauze Exit Temp.    950° C.                                            N-Loading           70 T (N)/m.sup.2 day                                      Gauze Cycle         81 Days                                                   ______________________________________                                    

Upon start-up, it was observed that the hydrogen torch could be removedafter only about 20 seconds while normally the torch is applied forabout 2 minutes. It was further observed qualitatively that the ignitionspread rapidly over the surface. Measured NO_(x) production is presentedin Table VI for the first five days of operation.

FIG. 5 illustrates the improvement in overall NO_(x) productiongraphically.

                  TABLE VI                                                        ______________________________________                                        Commercial Trial Results                                                      of Pt-Coated Gauze                                                                      Acid Production Rate (TPD)                                          On-Stream   Typical       Experimental                                        Time (Days) Conventional Gauze                                                                          Pt-Coated Gauze                                     ______________________________________                                        1           300           325                                                 2           302           324                                                 3           303           319                                                 4           304           314                                                 5           303           308                                                 6           303           303                                                 7           300           303                                                 8           302           302                                                 9           303           303                                                 10          301           301                                                 ______________________________________                                    

EXAMPLE VII

Catalyst gauzes prepared as set forth in Examples I were placed in thelaboratory ammonia oxidation reactor described above then 1% hydrogen inair passed through the gauzes of a flow rate of 8500 lb/ft² -hr with thetemperature being steadily increased until ignition was observed. Thisprocedure was repeated using 3% hydrogen. Table VII sets forth theignition temperature for each of these gauzes.

                                      TABLE VII                                   __________________________________________________________________________    Ignition Test with Hydrogen                                                              Pt-Loading                                                                          Surface Area*                                                                         Ignition Temp. (°C.)**                        Sample No. (g/m.sup.2)                                                                         (cm.sup.2 /g)                                                                         1% H.sub.2                                                                        3% H.sub.2                                       __________________________________________________________________________    I. Standard Gauze                                                             1          0     13      126 91                                               2***       0     13      103 79                                               II. Pt-Coated Gauzes                                                          3          6.26  62      71  66                                               4          5.60  93      52  43                                               5          4.50  497     25  less than 23                                                                  (Room Temp)                                      __________________________________________________________________________     *By Cyclic Voltammetry                                                        **At 9000 lbs/ft.sup.2- Hr. and 100 psig                                      ***(AcidWashed)                                                          

FIG. 3 illustrates the very low hydrogen ignition temperatures obtainedwith gauzes of this invention.

EXAMPLE VIII

Catalyst gauzes prepared as in Example I were tested for ignition atvarying gas throughputs as set forth in Table VIII which also presentsignition data for an ammonia feed concentration of 2.5% at 100 psigreactor pressure. Ignition temperature was reduced by 25° C. as massthroughput decreased from 10,320 to 2750 lb/ft² -hr. Further decreasesin the ignition temperature are expected for low/medium pressure plantconditions as illustrated in FIG. 2.

                  TABLE VIII                                                      ______________________________________                                        Effect of Throughput                                                          Throughput    Ignition                                                        Rate          Temperature                                                     (lb/ft.sup.2 -hr)                                                                           (°C.)                                                    ______________________________________                                        2,750         240 ± 2                                                      4,130         240 ± 2                                                      5,500         240 ± 3                                                      6,880         244 ± 4                                                      8,400         247 ± 2                                                      9,630         254 ± 1                                                      10,320        264 ± 3                                                      ______________________________________                                    

EXAMPLE IX

A series of platinum coated catalyst gauzes were prepared as describedin Example I except that the platinum precursor decompositiontemperature was varied between 400° and 600° C.

Table IX sets forth the results of characterization of these samplesincluding platinum loading surface area and the percentage of platinumnot fully converted to the metal from the precursor.

                  TABLE IX                                                        ______________________________________                                        Effect of Decomposition Temperature                                           Decomposition                                                                           Pt         Surface   Non-Metallic Pt                                Temp.     Loading    Area      Total Pt Coating                               ______________________________________                                        (°C.)                                                                            (g/m.sup.2)                                                                              (cm.sup.2 /g)                                                                           (%)                                            ______________________________________                                        400       7.5        2794-3383 23                                             450       6.3        696-2136  less than 1                                    500       5.6        567-586   0                                              550       5.5        267-366   0                                              600       5.5        183-240   0                                              ______________________________________                                    

Significant weight reductions occurred during precursor decompositionbelow 450° C., while increasingly severe sintering of the high surfacePt coatings was apparent for gauze samples wherein the precursor wasdecomposed above 450° C. The lowest light-off temperature was observedwith Pt-coated gauzes decomposed at around 450° C., as illustrated inFIG. 6.

EXAMPLE X

To demonstrate the feasibility of start-up from room temperature using astream of hydrogen in air instead of a hydrogen torch, catalyst gauzesprepared as set forth in Examples I were tested in the laboratoryreactor mentioned above by passing 1% hydrogen in air over the gauzewhich was initially at 23° C. until an exotherm was observed, theconcentration of hydrogen in the feed was then increased slowly until anexit temperature of 325° C. was observed whereupon ammonia initially ata concentration of 5% was passed over the gauzes until a furtherexotherm was observed whereupon the flow of hydrogen was discontinuedand the flow of ammonia increased to about 11% to verify thatsatisfactory ignition had been obtained.

EXAMPLE XI

After measuring the ignition temperature with 5% ammonia feed asdescribed in Example IV, the air preheat temperature was graduallyreduced until the ignition was extinguished. As the inlet temperaturedecreased, the gauze surface temperature started to cool down andeventually ignition could not be sustained. Accordingly, a minimum airpreheat temperature to sustain ignition was measured as the extinctiontemperature.

Table X sets forth the test results of the ignition and extinctiontemperatures for multiple steady states on the conventional andPt-coated gauzes at 8500 lb/ft² -hr throughput and 100 psig reactorpressure.

                  TABLE X                                                         ______________________________________                                        Hysteresis of Ammonia Oxidation Gauzes                                                     Surface Area                                                                             Ignition   Extinction                                 Sample No.   (cm.sup.2 /g)                                                                            Temp (°C.)                                                                        Temp (°C.)                          ______________________________________                                        Conventional Gauze                                                            X - 1        13         334        222                                        Pt-coated Gauze                                                               X - 2        364        252        66                                         ______________________________________                                    

As our invention, we claim:
 1. A method of forming a catalytic gauzeelement comprising forming a woven mesh consisting essentially ofplatinum, rhodium and optionally palladium, grounding said meshelectrically, and spraying electrically charged droplets comprising athermally decomposable platinum compound dispersed in a volatile liquidonto said mesh and thereafter evaporating said volatile liquid anddecomposing said thermally decomposable platinum compound to form a highsurface area coating of platinum upon said mesh, the surface area of theresulting coated mesh being at least about 50 cm² /g.
 2. The method ofclaim 1 wherein said volatile liquid is an aqueous liquid and thesurface area of the resulting coated mesh is at least about 250 cm² /g.3. The method of claim 2 wherein the surface area of the resultingcoated mesh is at least about 350 cm² /g.
 4. The method of claim 2wherein the surface area of the resulting coated mesh is at least about500 cm² /g.
 5. A catalytic element comprising filaments consistingessentially of platinum, rhodium and optionally palladium, saidfilaments having a high surface area coating consisting essentially ofplatinum formed thereon, the surface area of said catalytic elementexceeding 50 cm² /g.
 6. The catalytic element of claim 2 wherein thesurface area of said catalytic element exceeds 250 cm² /g.
 7. Thecatalytic element of claim 2 wherein the surface area of said catalyticelement exceeds 350 cm² /g.
 8. The catalytic element of claim 2 whereinthe surface area of said catalytic element exceeds 500 cm² /g.
 9. Thecatalyst of one of claims 5, 6, 7, or 8 wherein the weight of saidplatinum coating is at least about 0.5 grams per square meter of bulkgauze area.
 10. The catalyst of one of claims 5, 6, 7 or 8 wherein theweight of said platinum coating is at least about 3.0 grams per squaremeter of bulk gauze area.
 11. The catalyst of one of claims 5, 6, 7 or 8wherein the weight of said platinum coating is at least about 4.0 gramsper square meter of bulk gauze area.
 12. A method of forming a catalyticgauze element comprising forming a woven mesh consisting essentially ofplatinum, rhodium and optionally palladium, and spraying dropletscomprising a thermally decomposable platinum compound dispersed in avolatile liquid onto said mesh and thereafter evaporating said volatileliquid and decomposing said thermally decomposable platinum compound toform a high surface area coating of platinum upon said mesh, the surfacearea of the resulting coated mesh being at least about 50 cm² /g. 13.The method of claim 6 wherein said volatile liquid is an aqueous liquidand the surface area of the resulting coated mesh is at least about 250cm² /g.
 14. The method of claim 7 wherein the surface area of theresulting coated mesh is at least about 350 cm² /g.
 15. The method ofclaim 7 wherein the surface area of the resulting coated mesh is atleast about 500 cm² /g.
 16. A method of forming a catalytic gauzeelement comprising forming a woven mesh consisting essentially ofplatinum, rhodium and optionally palladium, and depositing a thermallydecomposable platinum compound dispersed in a volatile liquid onto saidmesh and thereafter evaporating said volatile liquid and decomposingsaid thermally decomposable platinum compound to form a high surfacearea coating of platinum upon said mesh, the surface area of theresulting coated mesh being at least about 50 cm² /g.
 17. The method ofclaim 16 wherein said volatile liquid is an aqueous liquid and thesurface area of the resulting coated mesh is at least about 250 cm² /g.18. The method of claim 17 wherein the surface area of the resultingcoated mesh is at least about 350 cm² /g.
 19. The method of claim 18wherein the surface area of the resulting coated mesh is at least about500 cm² /g.